Depth steerable seismic source array

ABSTRACT

A steerable seismic energy source includes at least one float. The floatation device includes a device for changing buoyancy thereof. A frame is coupled to the at least one float. At least one seismic energy source is suspended from the frame. At least one steering device is coupled to the floatation device or the frame. The at least one steering device includes at least one control surface and a control surface actuator coupled to the control surface. The actuator is configured to rotate the control surface to generate hydrodynamic lift at least in a vertical direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/655,062, filed Dec. 22, 2009, which is incorporated by referenceherein in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to the field of seismic surveying. Morespecifically, the invention relates to devices for navigating a marineseismic source array suspended in water from a floatation device toavoid navigation hazards.

2. Background Art

In marine seismic surveying, a seismic energy source is used to generateseismic energy in the form of acoustic pulses or waves in a body ofwater such a lake or the ocean. The seismic energy travels downwardly inthe water, through the water bottom, and through the Earth formationsunderlying the water bottom. Part of the energy passing through theEarth formations underlying the water bottom is reflected upward fromacoustic impedance boundaries in the Earth formations. The upwardlytraveling seismic energy is detected by sensors such as hydrophonestowed in one or more streamer cables disposed near the water surface, orby sensors disposed in cables along the water bottom. The sensorsconvert the detected energy to electrical or optical signals. Theelectrical or optical signals are then conditioned and interpreted toprovide information both as to the composition and the structure of thevarious subsurface Earth formations. Such information is usedparticularly to determine the possibility that such Earth formations maycontain mineral deposits such as hydrocarbons.

The most frequently used marine seismic energy source known in the artis an “air gun array.” A typical air gun array is a plurality ofindividual air guns of different sizes towed behind a seismic surveyvessel or a source vessel. The air guns are ultimately suspended from abuoy, float or similar flotation device. The flotation device istypically coupled to a frame or similar substantially rigid structure soas to suspend the frame in the water. Individual air guns forming thearray may be suspended from the frame by cables or chains of selectedlength so that the individual air guns are operated at a selected depthin the water. In air gun arrays known in the art, the floatation devicemay be steerable in the plane of the surface of the water, but remainsproximate the surface of the water by reason of the buoyancy of theflotation device.

SUMMARY OF THE INVENTION

A steerable seismic energy source according to one aspect of theinvention includes at least one floatation device. The floatation deviceincludes a device for changing buoyancy thereof. A frame is coupled tothe at least one floatation device. At least one seismic energy sourceis suspended from the frame. At least one steering device is coupled tothe floatation device. The at least one steering device includes atleast one control surface and a control surface actuator operativelycoupled to the control surface. The actuator is configured to rotate thecontrol surface to generate hydrodynamic lift at least in a verticaldirection.

A method for operating a seismic energy source in a body of wateraccording to another aspect of the invention includes suspending atleast one seismic energy source from a floatation device. The floatationdevice is towed in the water by a tow vessel. Buoyancy of the floatationdevice is reduced to cause submergence thereof in the event of anavigation hazard proximate the water surface.

Other aspects and advantages of the invention will be apparent from thefollowing description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example seismic acquisition system.

FIG. 2 shows the seismic source array of FIG. 1 in more detail.

FIG. 3 shows an example ballast control device.

FIG. 4 shows one example of an actuator for operating a control surfaceon the source array of FIG. 2.

FIG. 5 shows an alternative actuator for the control surface.

FIG. 6 shows another example seismic source that does not use areference buoy.

DETAILED DESCRIPTION

An example marine seismic data acquisition system that may be used witha seismic source according to the invention is shown schematically inFIG. 1. The acquisition system includes a seismic vessel 10 that movesalong the surface of a body of water 11 such as a lake or the ocean. Theseismic vessel 10 includes thereon equipment, shown generally at 12 anddescribed for convenience herein as a “recording system” that mayinclude (none shown separately in FIG. 1) data recorders, navigationdevices such as global positioning system (“GPS”) receivers and seismicenergy source control devices. The recording system 12 may also includeequipment for operating buoyancy control and hydrodynamic lift deviceson one or more seismic source arrays as will be explained below in moredetail.

The seismic acquisition system may include a plurality of seismic sensorstreamers 14 towed by the seismic vessel. 10 as shown in FIG. 1 or by adifferent vessel (not shown). The streamers 14 are generally cables thatextend behind the towing vessel for a certain distance, and each suchstreamer 14 includes seismic sensors 22 disposed thereon at spaced apartlocations. The seismic sensors 22 are typically pressure or pressuretime gradient responsive sensors such as hydrophones but may also beparticle motion responsive sensors such as accelerometers or geophones,or combinations of hydrophones and geophones. The type of sensor is nota limitation on the scope of the present invention. Geometry of thestreamers in the water is maintained by various towing devices includinglateral force generating devices called “paravanes” 18 disposed at theend of paravane lead in ropes 16. The streamers 14 are coupled at theirforward ends to a respective termination 21, which couples the streamer14 to a corresponding lead in cable 20. The paravane lead in ropes 16and lead in cables 20 may be deployed from the vessel 10 and retrievedthereon by winches (not shown) or similar spooling devices known in theart. The lateral separation of the paravanes 18 may be limited by aspreader cable 19 ultimately to maintain the geometry of the entirearray of streamers 14. In some examples the centermost section of thespreader cable 19 may be omitted.

The seismic vessel 10 may also tow, or another vessel (not shown) maytow one or more seismic source arrays 24. Only one such array is shownin FIG. 1 for clarity of the illustration. The seismic source array 24typically includes a plurality of seismic energy sources, which in thepresent example may be air guns having various chamber sizes, asexplained above. Upon suitably timed actuations of all the individualair guns in the array 24 (typically by control signals from therecording unit 12) a seismic energy pulse of particular spectral contentis imparted into the water 11. Seismic signals resulting from suchactuations are detected by the seismic sensors 22, and the detectedsignals are communicated to the recording system 12. The manner ofrecording and processing signals detected by the various seismic sensors22 is well known in the art and will not be further described herein.

The seismic source array 24 may be towed by the vessel 10 using anumbilical cable 113. The source array 24 may include a steering device26 proximate one or each longitudinal end thereof. The umbilical cable113 may include (none shown separately) a strength member to transfertowing force of the vessel 10 to the source array 24, one or morecompressed air or gas lines, and electrical and/or optical conductorsfor communication between various components of the source array 24 andthe recording system 12. As will be explained below, the steering device26 may provide steering capability to the source array 24 in bothlateral direction and in depth. In one example, the steering device 26may use a single wing or foil to perform steering in both vertical andhorizontal directions. The source array 24 may also include one or morefloatation devices (see FIGS. 2 and 3) that can provide controllablebuoyancy to the source array 24.

An example of the source array 24 is shown schematically in FIG. 2. Thesource array 24 may be towed by the survey vessel (10 in FIG. 1) fromthe aft end of the umbilical cable 113, as previously explained. Thesource array 24 in the present example includes a main keel or beam 112,which is typically coupled to the umbilical cable 113. The beam 112suspends a sub frame 115 at a selected distance therefrom using depthcontrol ropes 114. One or more seismic energy sources 119 (e.g., airguns) may be suspended from the subframe 115 by chains 118 or similarsuspension devices. Source depth determination may be performed bymounting depth (e.g., pressure) sensors 117 at a known vertical distancefrom the seismic energy sources 119. The number of seismic energysources in any particular implementation is not a limit on the scope ofthe present invention, nor is the invention limited to air guns. Theinvention may also be used with marine vibrators and water guns, as nonliming examples.

The main beam 112 can be connected to one or more floatation devices107, each of which consists of a volume of buoyant void filler, forexample, foamed styrene or foamed polyurethane enclosed in a sealedhousing (shown collectively at 107 a). The interior of each housing maydefine one or more liquid tight chambers 108 for ballasting with water.Thus, each floatation device 107 has a controllable buoyancy. Thepresent example shows three such floatation devices 107. Other examples,one of which will be explained below with reference to FIG. 6, mayinclude only a single such flotation device. The number of suchfloatation devices is a matter of discretion for the designer of asource array according to the various aspects of the invention and isnot intended to limit the scope of the invention. A possible advantageto using more than one floatation device as shown in the present exampleis to provide better control over leveling of the source array 24.

Ballasting can be controlled by a buoyancy regulating device 109. Thebuoyancy regulating device 109 (explained below with reference to FIG.3) can operate automatically, depending on measurements made from adepth (e.g., pressure) sensor 110 a, a load cell 106, and level or tiltsensors 110 b. A control surface 111 can be used to correct temporarydepth and/or level fluctuations. An example mechanism to operate thecontrol surface 111 will be explained with reference to FIG. 4. Thedimensions of the housing, void fill, housing material, and internalchambers 108 may be selected such that the floatation devices 107provide enough buoyant force to lift the floatation devices 107 to thewater surface when all water is expelled from the chamber(s) 108, andmay provide neutral or slight negative buoyancy when the chamber(s) 108are fully ballasted with water.

In some examples, a surface reference buoy 101 with a tail fin 104 forstabilizing movement in the water may be suspended from the front of theforwardmost one of the floatation units 107 by a line, cable or rope105. The rope 105 may be connected at one end to the load cell 106.Measurements made by the load cell 106 provide indication of how muchweight load is exerted by the front of the source array 24 on thereference buoy 101. Such weight indication can be used to adjust thefloatation ballast to correct the floatation of the source array 24 tomake it level. Measurements from the load cell 106 can also indicate ifthe reference buoy 101 has broken loose.

The other end of the rope 105 may be connected to a winch unit 102disposed in the reference buoy 101 for depth control of the sourcearray. The winch unit 102 may be controlled by a control andcommunication unit 103. The communication unit 103 may be in signalcommunication with the recording system (12 in FIG. 1) using, forexample a radio link. A GPS receiver 120 may be disposed on thereference buoy 101 for geodetic position determination. Other examplesmay omit the surface reference buoy 101. If, for example, it is desiredto increase the depth of the source array, a signal may be transmittedfrom the recording system (12 in FIG. 1) to the communication unit 103.The communication unit 103 will operate the winch 102 to extend the rope105.

Two or more handling cables 116 may be routed through the beam 112 andcorresponding guide blocks 121. The handling cables 116 extend fromfloatation units 107 to anchor point 122 on the umbilical cable 113. Theanchor point 122 may be located a selected distance forward of thefloatation units 107. The handling cables 116 may be used for deploymentand recovery of the source array 24.

An example of one of the buoyancy control devices 109 is shownschematically in FIG. 3. A controller 109 a, such as a microprocessormay accept as input measurements from one of the tilt sensors 110 b andthe depth (pressure) sensors 110 a. The controller 109 a may also be insignal communication with the recording system (12 in FIG. 1).

The forwardmost one of the buoyancy control devices 109, may in someexamples accept as input signals from the load cell 106. In the presentexample, some of the weight in water of the source array (24 in FIG. 2)may be supported by the reference buoy (101 in FIG. 2). The buoyantforce exerted by the reference buoy (101 in FIG. 2) will be measured bythe load cell 106. If the measured force is outside of a predeterminedrange, the ballasting can be adjusted by the buoyancy control device109. If the desired depth is not reached with the desired reference buoyload, the winch (102 in FIG. 2) must change the length of the deployedrope (105 in FIG. 2). Such change may be performed, for example, bycommunicating the load cell 106 measurement to the recording system (12in FIG. 1) over the umbilical cable (113 in FIG. 1), which will transmita signal to the reference buoy (101 in FIG. 2) to cause the winch (102in FIG. 2) to extend the rope (105 in FIG. 2).

Output of the controller 109 a may be coupled to a solenoid operatedthree-way pneumatic control valve 109 b. When signals from one or moreof the sensors (e.g., depth sensor 110 a, tilt sensor 110 b, and loadcell 106) indicate that the buoyancy should be increased, the controller109 a operates the valve 109 b to connect a source of compressed air orgas (not shown) to the chamber 108. Pressurized gas from the compressedgas source (not shown) may displace water in the chamber 108, therebyincreasing the buoyancy of the particular flotation device (107 in FIG.2). When the correct buoyancy has been attained, the controller 109 amay operate the valve 109 b to close the chamber 108, therebymaintaining the water level in the chamber 108. In case buoyancy isrequired to be decreased, the controller 109 a may operate the valve 109b to vent the chamber 108, so that water can enter the bottom of thechamber 108. The controller 109 a may be programmed to operate the valve109 b to maintain the floatation device (107 in FIG. 2) at selecteddepth in the water. The selected depth may be changed, for example, bycommunicating a control signal from the recording system (12 in FIG. 1)to the controller 109 a, for example, through a signal line (not shownseparately) in the umbilical cable (113 in FIG. 2). Such signal may beconverted in the controller 109 a into a signal to selectively operatethe pneumatic valve 109 b.

In one example, the valve 109 b is configured so that in the event ofelectrical or other component failure in the source array (24 in FIG.1), the default or failure mode position of the valve 109 b is toconnect the chamber 108 to the source of compressed gas or air. By suchconfiguration, the chamber(s) 108 will be automatically purged of waterand the source array (24 in FIG. 1) will be returned to the watersurface for retrieval in the event off component failure.

An example mechanism to operate the control surface 111 is shownschematically in FIG. 4. The present example uses an hydraulic actuator136 coupled by a suitable linkage 138 to the control surface 111. Theactuator may be operated by a solenoid operated valve 134 in signalcommunication with the controller 109 a. Hydraulic power to move theactuator may be provided by a pump 132 connected to an hydraulicreservoir 130. The controller 109 a may be programmed to operate thevalve 134, thereby operating the control surface 111 so that theassociated flotation device (107 in FIG. 2) is maintained at theselected depth, or is leveled in the event the source array (24 inFIG. 1) becomes unleveled (greater depth at one end than at the other).The control valve 134 may contain a hydraulic brake valve (not shownseparately) to lock the motion of hydraulic actuator 136, making thecontrol surface 111 resistant to being moved by force of moving thesource array through the water. Such a brake valve could be implementedusing, for example, using a three way valve such as shown above for thesolenoid operated valve 134. In a typical three way valve, the centerposition closes all valve ports, thereby stopping motion of hydraulicfluid into and out of the actuator 136.

Another example mechanism to operate the control surface 111 is shownschematically in FIG. 5. The controller 109 a may be in signalcommunication with a motor driver 142, which operate an electric motor140. The motor 140 may turn a worm gear 141. The worm gear may be incontact with a toothed sector 144. The sector 144 may rotate the controlsurface 111 in response to being moved by the worm gear 141. A possibleadvantage of the mechanism shown in FIG. 5 is that when the motor 140and worm gear 141 are not rotating, they act to lock the position of thesector 144, thus making the control surface resistant to being moved byforce of moving the source array through the water.

The foregoing example of a source array may be operated at any selecteddepth in the water by changing the buoyancy of the flotation devices(107 in FIG. 2) as explained above and by operating the control surface(111 in FIG. 2). The length of the rope or line (105 in FIG. 2) may beadjusted by the winch unit (102 in FIG. 2) to maintain a selectedtension on the line (105 in FIG. 2) as measured by the load cell (106 inFIG. 2), irrespective of the operating depth of the source array (24 inFIG. 1).

Other examples of the source array (24 in FIG. 1) may omit the referencebuoy (101 in FIG. 2). Such examples may be used in areas where it islikely to encounter ice or other navigation hazards on the surface ofthe water (11 in FIG. 1). In the event any such navigation hazard isencountered, the flotation devices (107 in FIG. 2) may be madenegatively buoyant by ballasting with water as explained above, and oneor more steering devices 26 may be operated to submerge and cause thesource array to move to greater depth in the water.

An example source array that may omit use of the reference buoy, anduses one or more steering devices (26 in FIG. 1) is shown in FIG. 6. Inthe present example of steering device there may be only one flotationdevice 107 coupled to the beam 112, having a longitudinal dimensionapproximately the same as that of the upper part of the beam 112. Thebeam 112 may have a steering foil or plane 150, which may be coupled toa mechanism 151 (including an actuator 151 a and corresponding linkage153) to cause rotation of the plane 150 along an axis perpendicular tothe direction of motion of the floatation device 107 (i.e., itslongitudinal dimension) and the beam 112 in the water. Such rotation ofthe plane 150 may cause lateral hydrodynamic lift as the source array(24 in FIG. 1) is moved through the water. Lateral hydrodynamic lift maybe used to position the source array in a selected position withreference to the towing vessel (e.g., vessel 10 in FIG. 1). The steeringdevice 26 may also include a mechanism including an actuator 154 and ahinge 152 coupled to the base of the deflecting mechanism 151 to enablethe plane 150 to rotate about an axis parallel to the direction ofmotion of the flotation device 107 and the beam 112. Such deflectionprovides hydrodynamic lift in a vertical direction. The actuators 151 aand 154 in the present example may be linear actuators, for example,hydraulic cylinder/ram combinations or electric linear actuators.Examples of the latter type of actuator may be obtained from LINAK A/SSmedevaenget 8, 6430 Nordborg, Denmark.

When it may be necessary to submerge the source array to avoid hazardson the water surface (such as ice), the flotation device 107 may beballasted with water, and the plane 150 may be operated, such as byoperating the actuator 154 to generate downward hydrodynamic force. Thesource array may be returned to the surface after the hazard has passedby removing the water ballast from the chamber (108 in FIG. 3) in thefloatation device 107, and the plane 150 may be operated by moving theactuator 154 so that the plane 150 generates no upward or downward lift,or generates upward lift until the source array flotation device 107 isat the water surface. The foregoing submersion of the source array mayalso be performed to cause the seismic sources (119 in FIG. 2) to beoperated at a selected water depth different from that obtained when theflotation device 107 is disposed proximate the water surface bybuoyancy.

Various examples of a marine seismic source according to the variousaspects of the invention may be able to operate at a selected depth forcertain seismic surveying purposes, and may be navigable to avoidhazards on the water surface. In some cases it may be desirable tosubmerge the entire source array, including the float, to operate theseismic energy sources at greater depth in the water. In some cases itmay also be desirable to be able to submerge the floatation device toavoid navigation hazards such as ice. Using source array structuresknown in the art prior to the present invention, it was generally notpossible to change the floatation device depth during operation of theseismic source array. A seismic source array according to the inventionmay be fully submerged to operate the seismic sources at greater waterdepth and to avoid surface navigation hazards such as ice.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

What is claimed is:
 1. A method, comprising: towing, by a tow vessel, afloatation device through a body of water, wherein the floatation deviceis coupled to at least one seismic energy source; and in response to anavigation hazard, reducing buoyancy of the floatation device to causethe floatation device to submerge.
 2. The method of claim 1, wherein thereducing buoyancy includes allowing water to enter one or more of aplurality of chambers of the floatation device such that the floatationdevice remains substantially level.
 3. The method of claim 1, furthercomprising adjusting a control surface coupled to the floatation deviceto generate a lateral force on the floatation device.
 4. The method ofclaim 1, further comprising: increasing buoyancy of the floatationdevice to cause the floatation device to at least partially emerge fromthe body of water; wherein the increasing buoyancy includes releasingpressurized gas to expel water from an internal chamber of thefloatation device.
 5. The method of claim 1, further comprising: inresponse to a failure event, increasing buoyancy of the floatationdevice to cause the floatation device to rise to a surface of the bodyof water.
 6. The method of claim 1, further comprising: determining aweight load on a particular area of the floatation device based on aforce imparted by a reference buoy coupled to the floatation device; andleveling the floatation device based on the determined weight load. 7.An apparatus, comprising: a seismic energy source; and one or morefloatation devices coupled to the seismic energy source and connectableto be towed by a tow vessel; wherein the apparatus is configured toreduce buoyancy of the floatation device by allowing water into multipleinternal chambers included in the one or more floatation devices; andwherein the apparatus is configured to increase buoyancy of thefloatation device by expelling the water.
 8. The apparatus of claim 7,wherein the one or more floatation devices include multiple floatationdevices that each include an internal chamber.
 9. The apparatus of claim7, further comprising: a reference buoy coupled to the floatationdevice; wherein the apparatus is configured to determine a weight loadon a particular area of the floatation device based on a force impartedby the reference buoy.
 10. The apparatus of claim 9, wherein theapparatus is further configured to level the floatation device based onthe determined weight load.
 11. The apparatus of claim 7, furthercomprising: an adjustable control surface; wherein the apparatus isconfigured to adjust the control surface to steer the floatation device.12. The apparatus of claim 7, wherein the apparatus is configured tooperate in a default mode in which the apparatus is configured to causethe floatation device to lift to the surface of a body of water.
 13. Theapparatus of claim 7, wherein the apparatus is configured to reducebuoyancy of the floatation device in response to a navigational hazardproximate to a surface of a body of water.
 14. An apparatus, comprising:an energy source configured to impart energy to a geophysical formationduring a geophysical survey; a floatation device coupled to the energysource and connectable to be towed by a tow vessel; and a control unitconfigured to reduce buoyancy of the floatation device to adjust a depthof the energy source in a body of water, wherein the control unit isconfigured to reduce the buoyancy of the floatation device in responseto an indication of a navigational hazard.
 15. The apparatus of claim14, wherein the apparatus is configured to operate in a default mode inwhich the apparatus is configured to cause the floatation device to liftto the surface of a body of water.
 16. The apparatus of claim 14,wherein the control unit is configured to allow water to enter aninternal chamber of the floatation device to reduce the buoyancy of thefloatation device.
 17. The apparatus of claim 16, wherein the apparatusis configured to expel the water to increase buoyancy of the floatationdevice.
 18. The apparatus of claim 17, further comprising: a three-wayvalve; wherein the apparatus is configured to expel the water using acompressed gas and the three-way valve.
 19. The apparatus of claim 14,further comprising: an adjustable control surface; wherein the controlunit is configured to adjust the control surface to steer the floatationdevice.